A wide-gap semiconductor such as Gallium Nitride (GaN) or Silicon Carbide (SiC) has better characteristics such as a high critical electrical field and thermal conductivity as compared to Silicon (Si), and is being actively studied and developed in recent years to put into production for power devices. Particularly, GaN based high electron mobility transistors (HEMTs) such as AlGaN/GaN HEMTs with two-dimensional electron gas channel have a high electron mobility and carrier density. This allows GaN devices to realize good high frequency characteristics and low on-state resistance as compared to Si devices and receive a great expectation as one of the most promising candidates for future switching power device for power electronics applications.
Meanwhile, a GaN device having a typical AlGaN/GaN heterojunction channel is a normally on type in which a gate threshold voltage is a negative voltage, and has a concern that the gate electrode is broken down easily when an excessive positive voltage is applied to the gate electrode. In order to avoid breaking down of the gate electrode, a diode which clips the gate voltage to protect the gate electrode is generally provided between a gate and source outside a chip, and therefore there are problems that the number of parts of a circuit increases and interconnections on a circuit substrate become complicated.